Despite significant recent advances in the biochemistry of contractile proteins and the regulation of muscle contraction very little is known about myosin linked regulation in mammalian striated muscle, particularly cardiac muscle. Our initial studies show that the 18,000 M.W. subunit of cardiac myosin (Lc2) acts as an inhibitor of actin - myosin interaction. This suggests the possibility of a myosin linked control of actin - myosin interaction (contractile event) in mammalian cardiac muscle. This project will study the effects of removal of the cardiac myosin Lc2 on the control of actin-myosin interaction as determined by measurement of actin-activated Mg2 ion ATPase. The studies will be done using pure actin and regulated actin as cofactors. With pure actin as cofactor we shall study the effects of Ca2 ion on the affinity of myosin for actin in the presence and absence of the light chain. We shall study the mechanisms that reverse the (Lc2) inhibition of actin-myosin interaction. Attention will be focused on the effects of binding of phosphorylated and dephosphorylated Lc2 to the light chain deficient myosin in relation to measurements mentioned above. Hybridization of the Lc2 deficient cardial myosin with the homologous light chains from skeletal, smooth muscle and molluscan myosin will be performed and the effects of Ca2 ion and/phosphorylation on the actin-myosin interaction studied. In vivo studies (James Scheuer, Montefiore Hospital) will be done in isolated perfused rat hearts. Effects of various interventions (increased or decreased contractility) on the state of phosphorylation of myosin (Lc2) will be determined. It is expected that these studies will provide unique basic information regarding the role of myosin in regulating the contraction of cardiac muscle.